@_specializeがもっと書きやすければいいのだけど。ジェネリックパラメータが多いと大変。

extension StringProtocol { @usableFromInline @_specialize(where Self == String, S == String, T == String) @_specialize(where Self == String, S == String, T == Substring) @_specialize(where Self == String, S == Substring, T == String) @_specialize(where Self == String, S == Substring, T == Substring) @_specialize(where Self == Substring, S == String, T == String) @_specialize(where Self == Substring, S == String, T == Substring) @_specialize(where Self == Substring, S == Substring, T == String) @_specialize(where Self == Substring, S == Substring, T == Substring) …

(edited)